This invention relates to the suitable making up or covering of offset printing cylinders. Offset printing is also called lithographic printing. The invention in certain aspects extends to two piece printing elements in general.
In the past offset printing blankets have been one piece printing blankets having a strong dimensionally stable sublamina or laminas overlaid with compressible lamina or laminas overlaid with an additional strong dimensionally stable lamina which in turn has been overlaid with an ink transfer lamina. One embodiment of such an offset printing blanket is depicted in FIG. 2 as prior art. An edge portion of the offset printing cylinder 10 is shown at the edge of the lock-up gap 11. The blanket is illustrated, extending down into the gap to a lock-up mechanism (not shown) which holds the blanket taut across the cylindrical surface 12 of the offset printing cylinder 10. The blanket as shown has a substantially inextensible woven cotton fabric cylinder ply or lamina 13 which is nominally 13 mils thick, overlaid by a neoprene adhesive lamina 14 which is nominally 1 mil thick, which in turn is overlaid by another substantially inextensible woven cotton fabric lamina 15 which is also nominally 13 mils thick, overlaid with a nitrile adhesive layer 16 which is also nominally 1 mil thick, overlaid by a compressible layer 17 which is nominally 25 mils thick and was made according to U.S. Pat. No. 3,147,698 which is incorporated herein by reference. This compressible layer is overlaid by another nitrile adhesive lamina 18 having a nominal thickness of 1 mil which in turn is overlaid by an extensible nylon fabric lamina 19 having a nominal thickness of 6 mils, and this is overlaid by a nitrile adhesive layer 22 aggressive to nylon having a nominal thickness of 1 mil which in turn is overlaid by an ink transfer working face which is made up of a stiffer under lamina 20 which is nominally 11 mils thick overlaid by the ink transfer face 21 which is also nominally 11 mils thick. The total offset printing blanket thickness of the illustrated embodiment is 82-83 mils and, as already stated, it is of one piece construction. The thickness values given are nominal because each layer varies somewhat in thickness. The described offset printing blanket is sold by W. R. Grace & Co. as POLYWEB* NP Web offset newspaper blankets. Trademark of W. R. Grace & Co.
Because each end of the offset printing blanket must be secured in the lock-up gap of the offset printing cylinder the gap in the cylinder's circumference must be 1/4-3/4 inch wide because each end of the blanket as it enters the gap is 82 mils and the ends generally contain bulky lockup bars. The dimensions of the bars can be reduced commensurate with the reduced thickness and bulk of the blanket portion being secured in the gap as will be obvious to those skilled in the art. If the blanket could be thinned down where it enters the gap, the gap could be narrowed and this would increase the number of printing lines that a cylinder of a given circumference can accommodate. The area of a cylinder circumference taken up by the gap will not accommodate printing lines. The question is how to do this in a manner that will allow long continuous runs and yet retain the high quality printing demanded in offset printing.
Two-piece packing blankets have been known for many years in the newspaper printing art and in the DILITHO* printing art. These two-piece blankets are not involved in the transferring of the printing indicia to the paper but instead are involved in providing a cushion behind the paper when the type is impinged against the paper from the opposite side. Therefore, of course, none of them has the ink transfer working surface. In other aspects of their actual construction detail they are, however, very similar to the two-piece offset printing blanket of the present invention.
In fact, the bottom or compressable layer of the Example of this patent application showing the present invention is the identical material in physical structure to the structure sold by W. R. Grace & Co. as Polyfibron* type NY letterpress packing blanket for Nyloprint* letterpress application which is illustrated in FIG. 3. It has a removable polyethylene release sheet 25, a pressure sensitive adhesive layer 26 which is nominally 2 mils thick, a substantially inextensible woven cotton fabric cylinder ply 27 such as the one described above but having a nominal thickness of only 12 mils, overlaid with a nitrile rubber adhesive 28 having a nominal thickness of 2 mils, overlaid with a compressible layer 29 of the same description given above and having a nominal thickness of 25 mils. The compressible layer 29 is overlaid with a thermosol stabilizing layer 30 that is nominally 9 mils thick.
In letterpress printing the two piece packing's covering element is called a drawsheet. A typical drawsheet is sold by W. R. Grace & Co. as POLYDRAW* newspaper letterpress blanket. Such a drawsheet is shown in FIG. 4. The drawsheet illustrated in FIG. 4 is 33 mils thick and is constructed with a stabilizing cotton fabric ply 31 such as described above that is nominally 15 mils thick that is overlaid with a nitrile adhesive ply 32 that is aggressive to cotton fabric and is nominally 1 mil thick, overlaid with a nitrile adhesive ply 33 that is aggressive to nylon and nominally a half mil thick, overlaid with smooth plain weave nylon fabric 34 that is nominally 4 mils thick woven of nylon multifilament yarns having a denier of 70 with 100 yarns in the warp direction and 90 yarns in the fill direction. The nylon lamina is overlaid with another lamina of nitrile adhesive 35 that is aggressive to nylon and nominally a half mil thick and this in turn is overlaid with a rubber face layer 36 that is nominally 9 mils thick and is not suitable as a transfer surface but instead is suitable in use for backing up paper that is being printed on its exposed surface. *Trademark of ANPA Research Institute, W. R. Grace & Co., BASE and W. R. Grace & Co., respectively.
It is an object of the present invention to provide a method of making-up offset printing cylinders that will enable the reduction in the cylinder lock-up gap thereby increasing the lines of print that a cylinder of a given circumference can accommodate.
A further object is to provide a method of making-up offset printing cylinders that improves the print quality.
Yet another object of the invention is to provide an offset printing blanket that meets both of the objectives given above.